Another choice is to use their phone number. They sign up on the web, type in their phone number. Then they are shown a PIN, you have an automated phone system

Another choice is to use their phone number. They sign up on the web, type in their phone number. Then they are shown a PIN, you have an automated phone system that dials the phone number and prompts them to enter the PIN that is displayed. RapidSSL.com uses this method to quickly confirm a user for their SSL certificate, so I assume it is good enough for your use.